To provide a liposome and a liposome composition in which release of a drug in blood is suppressed during accumulation in a target site such as a tumor and the drug is rapidly removed from the blood after sufficient accumulation in the target site.SOLUTION: According to the present invention, there is provided a liposome containing at least a phospholipid to which a hydrophilic polymer is bonded, a phospholipid having a negative charge in the pH range from 6 to 8, and a phospholipid which is electrically neutral in the pH range from 6 to 8, in which an average carbon chain length of the phospholipid to which the hydrophilic polymer is bonded is shorter than an average carbon chain length of the phospholipid which is electrically neutral in the pH range from 6 to 8.SELECTED DRAWING: None【課題】本発明の課題は、腫瘍などの標的部位に集積する間は血中での薬物の放出が抑制され、かつ標的部位に十分に集積した後に速やかに薬物が血中から除去されるリポソーム及びリポソーム組成物を提供することである。【解決手段】本発明によれば、親水性高分子が結合したリン脂質、ｐＨ６～８の領域で負電荷を持つリン脂質、及びｐＨ６～８の領域で電気的に中性であるリン脂質を少なくとも含むリポソームであって、上記の親水性高分子が結合したリン脂質の平均炭素鎖長が、上記のｐＨ６～８の領域で電気的に中性であるリン脂質の平均炭素鎖長よりも短い、上記リポソームが提供される。【選択図】なし
